Geometric and Electronic Confinement Effects on Catalysis

We report a density functional theory-based study of the electronic and geometric effects exerted on molecules confined between transition-metal surfaces. We first investigate changes in adsorption energies and activation barriers for dissociation of O-2 and NO2 on Pt(111) and Fe(111), respectively. It is found that the energy barrier for dissociation of an O-2 molecule adsorbed on the bridge site when the molecule is confined between two Pt (111) surfaces is 35% lower than that on a single surface, and the barrier becomes negligible when the molecule adsorbs with its axis forming and angle of 60 or 90 degrees with the planes of the surfaces. In the case of NO2 confined between Fe (111) surfaces, the decrease is from 5 to 12% in the first deoxygenation and 10-15% in the second deoxygenation reaction. For both confined molecules, there is a substantial charge transfer from the metal surfaces that induces bond weakening, thus facilitating dissociation. It is also found that the dissociations of CO, NO, and O-2 on various confined transition-metal surfaces follow the well-known linear Bronsted-Evans-Polanyi relationships between adsorption and activation energies, yielding smaller slopes than the same reactions on single surfaces, thus evidencing facilitation of the catalytic reaction. These findings suggest that the reported phenomena could be useful for the design of nanoporous catalytic structures as well as sensors and nanoelectronic devices.